An optical film where a liquid crystal compound is highly oriented and fixed is recently overspread in various uses such as optical compensating film of liquid crystal display devices, brightness enhancing film, and optical correction film of projection display devices. In particular, the progress as an optical compensating film of liquid crystal display devices is remarkable.
The liquid crystal display device comprises polarizing plates and a liquid crystal cell.
In a TN-mode TFT liquid crystal display device which is predominating at present, an optical compensating sheet is interposed between a polarizing plate and a liquid crystal cell to realize a liquid crystal display device having high display grade. However, this method has a problem that the thickness of the liquid crystal display device itself becomes large.
JP-A-1-68940 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses an invention where an elliptically polarizing plate comprising a polarizing film having a retardation plate on one surface and a protective film on another surface is used and thereby the front contrast can be elevated without increasing the thickness of the liquid crystal display. However, the retardation film (optical compensating sheet) of this invention cannot provide a sufficiently high effect of improving the view angle and the display grade of the liquid crystal display device disadvantageously decreases.
JP-A-7-191217 and EP-A-0911656 disclose an invention where an optical compensating sheet comprising a transparent support having provided thereon an optically anisotropic layer formed of a discotic (disc-like) compound is used directly as the protective film of the polarizing plate and thereby the problem regarding the view angle is overcome without increasing the thickness of the liquid crystal display device.
In conventional techniques, the optical compensating sheet has been studied by mainly taking account of a small- or medium-size liquid crystal display device of 15 inches or less. However, in recent years, a large-size liquid crystal display device of 17 inches or more having high brightness must be also taken into consideration.
When an optical compensating sheet according to conventional techniques is fit as the protective film on a polarizing plate of a large-size liquid crystal display device, unevenness is generated on the panel. This defect is not so distinct in a small- or medium-size liquid crystal display device. However, with the progress of a large-size liquid crystal display device having high brightness, it becomes necessary to further develop an optical film coping with the unevenness due to light leakage.
JP-A-11-148080 discloses a technique of improving the unevenness by incorporating a leveling agent into the polymerizable liquid crystal. However, this is found to be effective only when the polymerizable liquid crystal is oriented in homogeneous alignment, but cannot be applied to complicated orientations including hybrid alignment.
The optical compensating sheet using the discotic liquid crystalline molecule is generally produced by dissolving a discotic liquid crystalline molecule in a certain organic solvent to prepare a coating solution and coating the solution. However, this method has a problem that unevenness ascribable to the process from coating to drying is generated with high frequency and a sheet having uniform optical properties over a wide size range can be hardly ensured, as a result, the yield greatly decreases.
The generation of this unevenness is caused by the fluctuation in the layer thickness of coated layer, brought about due to uneven drying. Therefore, a method of adding a certain surface active agent to a coating solution of discotic liquid crystalline molecule to improve the coatability of coating solution and thereby inhibit the layer thickness from fluctuating at the drying is disclosed (see, for example, JP-A-9-230143 and JP-A-2001-330725). However, in this method, the orientation of surface active agent slowly proceeds to the air-liquid interface and the drying is completed before the coatability of coating solution is improved. Thus, a sufficiently high improving effect cannot be obtained.